1. Field of the Invention
The present invention relates to thermal transfer elements and imaging radiation addressed (e.g., laser addressed) thermal mass transfer processes for use in the manufacture of high resolution emissive arrays. More particularly the invention relates to the use of radiation addressable thermal transfer elements having emissive materials such as phosphors in the transfer layer.
2. Background of the Art
Historically, phosphor arrays have been used in a variety of products including televisions, personal computer (PC) monitors, medical devices, oscilloscopes, radar tubes, optoelectronic image converters, personal safety products, bar coding, medical imaging screens (intensifying or storage phosphor screens), etc. Emissive arrays and phosphor display technology is expanding with the introduction of emissive flat panel display devices such as field emission displays (FEDs), electroluminescent displays (ELs), plasma displays (PDPs), vacuum fluorescent displays (VFD's), etc. A review of emissive display technology is provided in the Society for Information Display's publication Fundamentals of Emissive Technology by C. Curtin and C. Infante. As emissive display technology spreads into related product areas, the market continues to demand higher quality and higher resolution products. For example, miniaturized display devices for use in televisions, PC's, and camcorder viewfinders require a resolution of more than 50 lines/mm. (Oki, K. and L. Ozawa, "A Phosphor Screen for High-Resolution CRTs," Journal of the SID, 3, 51, 1995). For high-definition projection televisions having large picture formats, the requirements for the diameter of the electron spot is about a tenth of the diameter of the spot in present direct-view cathode ray tube screens and the maximum energy excitation density (approximately 2 W/cm.sup.2) is about a hundred times higher. (Raue, R., A. T. Vink and T. Welker, Philips Tech. Rev., 44, 335, 1989). These performance standards are very difficult to achieve with the current phosphor screen methods of manufacture, even though the phosphors available have the theoretic capability of providing these characteristics.
Phosphors are a critical component of cathode-ray tubes (CRTs), field electroluminescence devices (commonly referred to as EL devices), plasma display panels (PDP), light emitting diodes (LEDs), and field-emitting displays (FEDs).
In CRTs the quality of the screen image is dependent upon the cathodoluminescent efficiency and resolution of the phosphor screen. Many methods exist for the production of phosphor screens. A review of the various methods and their applications is described in Hase, T., T. Kano, E. Nakazawa, and H. Yamamoto, "Phosphor Materials for Cathode-Ray Tubes," Advances in Electronic and Electron Physics, Academic Press, Inc., New York, 79, 271 (1990).
Traditionally, the sedimentation process has been and still is the predominant process for depositing phosphors onto screens for monochrome CRTs. In this process, a suspension of the phosphor in alcohol or water, with the addition of an aqueous silicate solution, is placed in the glass envelope or bulb of the CRT and is allowed to deposit onto the inner surface of the faceplate through sedimentation. The phosphor forms a layer whose adhesion, both to itself and to the glass, is effected by the slowly precipitating silicic acid. The coagulation time of the aqueous silicate is adapted to the sedimentation rate of the phosphor by addition of electrolytes. The resultant screen has a relatively rough surface having phosphor particles that are loosely packed due to the coagulation process. Even though the loosely packed phosphor screen may have a somewhat higher cathodoluminescent efficiently than screens having more closely packed phosphors, the resolution of the loosely packed screens is lower. Another disadvantage of this method is that it requires relatively thick (approximately 6 mg/cm.sup.2) screens to insure a pinhole free coating, which thickness also decreases the resolution capability of the screen.
A slurry method is typically used in the production of shadow mask and aperture grill color CRTs where the screen consists of an array of multicolored dots or stripes. In this process, a slurry of a single color phosphor in a photosensitive resin is initially spin-coated onto the glass panel as a continuous layer. The coating is exposed to ultraviolet (UV) radiation from a point source through the apertures of a shadow mask, thus rendering the exposed areas insoluble in water. The non-exposed areas are removed by washing with water to form a phosphor image on the glass panel. This imaging process is then repeated at least two more times using phosphors of different colors to generate green, blue and red phosphor patterns. A dusting method is also sometimes employed to manufacture multicolored shadow mask CRTs. In the dusting method, the same basic process is used as described above except that dry phosphor is dusted onto the wet photosensitive coating prior to imaging. Exposure of the screen by UV radiation through shadow mask apertures immobilizes the phosphor coating in the irradiated areas. This process is then repeated until all three colored phosphor patterns are formed on the glass panel. The primary concerns with these methods is the trade-off between pinhole formation and contamination by other color phosphors in the wash-off step. If a strong rinse is used, pinholes may form and if a weak rinse is used, the color phosphors may not be completely washed away in the non-exposed areas. An alternative dusting method uses a phototackifiable resist. In this method, the photosensitive layer is exposed with UV radiation prior to depositing the phosphor. The phosphor adhere to only the tackified image areas. Again, the primary concern with this method is contamination by other color phosphors.
For applications requiring highly dense monochromatic phosphor screens with small particles, a deposition method is typically used. In this process, the phosphor powders are suspended in a polar organic solvent and cationic additives are adsorbed onto the surface of the phosphor. A negative potential is applied to a conductive substrate immersed in the solution with respect to a negative electrode held parallel to the substrate. The resulting applied electric field causes the positively charged phosphor particles to migrate to the substrate, thus coating the surface.
One such application requiring a high density phosphor screen is medical X-ray imaging. These screens usually comprise phosphors in a binder on a carrier layer. The phosphors absorb X-ray radiation at a higher efficiency than does silver halide which is normally used in the hard-copy output of radiographic images. The phosphors not only absorb X-rays at an efficient rate, but can also phosphoresce, emitting radiation at a wavelength other than the wavelength of X-rays which the phosphor absorbed. Depending upon the chemical nature and properties of the phosphor, the emitted radiation may be at essentially any wavelength between and including the infrared and ultraviolet wavelengths of the electromagnetic spectrum. Silver halide naturally absorbs radiation in the ultraviolet and near blue wavelengths, and can be spectrally sensitized to efficiently absorb radiation in other portions of the ultraviolet, visible and the infrared regions of the electromagnetic spectrum. By exposing the phosphor screen to X-rays, having the phosphor screen emit in the UV, visible or infrared, and having a silver halide emulsion spectrally sensitized to the wavelength of emission of the phosphor screen and optically associated with the phosphor screen, the entire efficiency of the X-ray imaging system can be greatly enhanced. This allows for the use of lower doses of X-rays during exposure of the object.
The use of such phosphors is well known in the art as exemplified by such patents as U.S. Pat. Nos. 3,883,747 and 4,204,125 where there is direct emission of phosphorescent radiation upon X-ray stimulation, and U.S. Pat. Nos. 3,859,527 and 5,164,224 where there is exposure to X-rays, storage of the absorbed energy by the phosphor, and subsequent stimulation by stimulating radiation to cause the phosphor to emit the stored energy as UV to infrared radiation. These phosphor systems are commercially successful and provide a significant benefit to the radiographic art. In these types of systems, however, there is a trade-off between speed and sharpness. To absorb more X-rays and emit more light, the screen itself can be made thicker. But in this case, light generated within the thickness of the screen is scattered by the phosphor grains to a greater extent, thereby reducing the resulting image sharpness recorded on the film. Conversely, to improve sharpness a thinner screen is desirable, but this reduces the X-ray absorbing power, and ultimately requires a higher dosage to the patient or object being X-rayed.
Many methods of improving the image quality, particularly the sharpness of images generated from phosphor screens, without adversely affecting the sensitivity or speed of the system, have been proposed. Reflective particulates, dyes, pigments and other light affecting materials have been proposed as additives to phosphor layers to improve sharpness as shown in EPO 102 790 (powdered glass), Japanese Application 146,447/1980 (white pigments), Japanese Patent Application 16-3,500/1980 (colorants), and EPO 175 578 (sputtering or vacuum evaporation of phosphors).
The objective of these methods primarily is to provide a high concentration of phosphor in the active layer of the screen and provide a screen of uniform properties. U.S. Pat. No. 5,306,367 produces a storage phosphor screen by dispersing phosphor particles in a thermoplastic binder diluted with a solvent, then coats the mixture, dries to remove the solvent, and compresses the coating at a temperature above the melting point of the binder. U.S. Pat. No. 5,296,117 deposits phosphor particles in a binder by electrophoretic deposition of a dispersion of the phosphor particles in a solution of polymeric binder. The solution is coated onto a substrate, dried and the phosphor screen thus produced. Each of these types of systems has shown some benefits, but there is still significant room for improvement in the sharpness of radiographic phosphor screens. In particular, it is desired to eliminate complicated deposition processes which can be costly, to eliminate the use of solvents which are harmful to the environment, and to eliminate or reduce high processing temperatures.
Some attempts have been made to provide a method of transferring a phosphor image directly onto a glass panel using a thermal transfer tape, ribbon or sheet and a thermal head printer. Examples of this type of application are disclosed in Japanese Application Nos. 63-02270A; 62-67416A; and 84-020466B. The advantage of this type of method is the selective placement of the phosphor on the substrate. However, the use of thermal printer heads limits the composition, shape and configuration of substrate used, produces low resolution images limited by the size of the printhead, makes the registration of adjacent phosphors difficult to control, and reduces the through-put of manufactured materials because of the slow speed of printheads. For example, the substrate must be flat to achieve a uniform transfer of the image. In addition, thermal print heads are currently limited in size and face a practical limit in reducing the size of the printing head.
Japanese Patent Application No. 62-95670A describes a thermal transfer construction which uses a conductive film layer within the construction. The transfer element is imaged by means of electrodes installed over the element. This construction suffers the same limitation as the conventional thermal transfer elements in that the substrate must be flat to achieve uniform transfer of the image.
There is a need for an efficient dry process for forming an emissive material or phosphor image on a variety of substrate sizes and configuration. In addition, there is a need for materials that are capable of producing high resolution and large excitation density to meet the increasing demands in the manufacture of high-definition televisions, field emission displays, and other hybrid display techniques.
The increasing availability and use of higher output compact lasers, semi-conductor light sources, laser diodes and other radiation sources which emit in the ultraviolet, visible and particularly in the near-infrared and infrared regions of the electromagnetic spectrum, have allowed the use of these sources as viable alternatives for the thermal printhead as an energy source. The use of a radiation source such as a laser or laser diode as the imaging source is one of the primary and preferred means of transferring electronic information onto an image recording media. The use of radiation to expose the media provides higher resolution and more flexibility in format size of the final image than the traditional thermal printhead imaging systems. In addition, radiation sources such as lasers and laser diodes provide the advantage of eliminating the detrimental effects from contact of the media with the heat source. The size, shape, energy and duration of the spot dwell time may be readily controlled according to the needs of the particular process and materials used. Various thermal imaging materials and processes are shown in U.S. Pat. Nos. 5,171,650, 5,156,938, GB Patent Application 2 083 726 A and Japanese Kokai Patent Publication Sho 63[1988]-60793.
U.S. Pat. Nos. 5,171,650 and 5,156,938 disclose an information transferring system and process in which materials are propulsively transferred from a donor layer to a receptor layer. Amongst the many materials listed which could be transferred in this information transferring system are luminescent materials (U.S. Pat. No. 5,171,650, column 13, lines 8-23) and phosphors (e.g., U.S. Pat. No. 5,278,023). The phosphors are included within the broad class of materials which provide information density when transferred, and although described as the types of phosphors used for television or medical imaging purposes, are not transferred to coat an entire surface, but are to be distributed in an information bearing pattern.
U.S. Pat. No. 5,171,650 discloses methods and materials for thermal imaging using an "ablation-transfer" technique. The donor element used in the imaging process comprises a support, an intermediate dynamic release layer, and an ablative carrier topcoat containing a colorant. Both the dynamic release layer and the color carrier layer may contain an infrared-absorbing (light-to-heat conversion) dye or pigment. A colored image is produced by placing the donor element in intimate contact with a receptor and then irradiating the donor with a coherent light source in an imagewise pattern. The colored carrier layer is simultaneously released and propelled away from the dynamic release layer in the light struck areas creating a colored image on the receptor.
European Patent No. EP 562,952 discloses ablative imaging elements comprising a substrate coated on a portion thereof with an energy sensitive layer comprising a glycidyl azide polymer in combination with a radiation absorber. Demonstrated imaging sources included infrared, visible, and ultraviolet lasers. Solid state lasers were disclosed as exposure sources, although laser diodes were not specifically mentioned. This application is primarily concerned with the formation of relief printing plates and lithographic plates by ablation of the energy sensitive layer. No specific mention of utility for thermal mass transfer was made.
U.S. Pat. No. 5,308,737 discloses the use of black metal layers on polymeric substrates with gas-producing polymer layers which generate relatively high volumes of gas when irradiated. The black metal (e.g., black aluminum) absorbs the radiation efficiently and converts it to heat for the gas-generating materials. It is observed in the examples that in some cases the black metal was eliminated from the substrate, leaving a positive image on the substrate.
U.S. Pat. No. 5,278,023 discloses laser-addressable thermal transfer materials for producing color proofs, printing plates, films, printed circuit boards, and other media. The materials contain a substrate coated thereon with a propellant layer wherein the propellant layer contains a material capable of producing nitrogen (N.sub.2) gas at a temperature of preferably less than about 300.degree. C.; a radiation absorber; and a thermal mass transfer material. The thermal mass transfer material may be incorporated into the propellant layer or in an additional layer coated onto the propellant layer. The radiation absorber may be employed in one of the above-disclosed layers or in a separate layer in order to achieve localized heating with an electromagnetic energy source, such as a laser. Upon laser induced heating, the transfer material is propelled to the receptor by the rapid expansion of gas. The thermal mass transfer material may contain, for example, pigments, toner particles, resins, metal particles, monomers, polymers, dyes, or combinations thereof. Also disclosed is a process for forming an image as well as an imaged article made thereby.
Laser-induced mass transfer processes have the advantage of very short heating times (nanoseconds to microseconds); whereas, the conventional thermal mass transfer methods are relatively slow due to the longer dwell times (milliseconds) required to heat the printhead and transfer the heat to the donor. The transferred images generated under laser-induced ablation imaging conditions are often fragmented (being propelled from the surface as particulates or fragments).